Stephen Sondheim’s brilliant work is a cynical, sentimental, scabrous sex comedy in which couples fall noisily apart and re-configure themselves, possibly for the better. The lovers’ struggles provoke plenty of witty commentary from the characters who are too young, too old, or too wise to participate in the madness. Ryan Turner conducts and Lynn Torgove directs this semi-staged production of one of the great 20th century operettas.